1. Field of the Invention
This invention relates to a process and apparatus for combustion of waste such as municipal solid waste (MSW), refuse derived fuel (RDF) or other comparable solid waste; the process results in simultaneous reduction in nitrogen oxides (NO.sub.x), carbon monoxide (CO), total hydrocarbons (THC), dioxins (PCDD), furans (PCDF), and other organic emissions.
2. Description of the Prior Art
Most of the existing processes and apparatuses for combustion of waste include a combustion chamber equipped with a sloped or horizontal stoker grate that reciprocates or travels to move the waste from the waste inlet side of the combustor to the ash removal side of the combustor. A portion of the combustion air, generally equivalent to 1.0 to 1.3 of the waste stoichiometric requirement, is supplied under the stoker grate. Such combustion air is typically called undergrate air, or UGA, and is distributed through the stoker grate to dry and burn the waste present on the stoker grate. The waste is first dried on the drying portion or drying grate of the stoker grate, then combusted on the combustion portion or combustion grate of the stoker grate. The residual waste that primarily includes ash and carbon is then decarbonized or burned on the burnout portion or burnout grate of the stoker grate. The bottom ash is then removed through an ash pit. To assure carbon burnout, a high level of excess air, compared to the amount required for carbon burnout, is maintained at the burnout grate. In addition to other species, the products of waste drying, combustion and burnout contain products of incomplete combustion (PIC's) such as carbon monoxide (CO) and total hydrocarbons (THC), oxides of nitrogen (NO.sub.x), such as NO, NO.sub.2, N.sub.2 O and other nitrogen bearing compounds such as NH.sub.3, HCN and the like.
The majority of NO.sub.x evolved from the stoker grate is believed to form from the oxidation of nitrogen bearing compounds and a smaller portion forms from the oxidation of molecular nitrogen.
Additional air or overfire air is usually introduced above the stoker grate and mixed with the products evolved from the stoker grate to burn out the combustibles. The excess air level downstream of the overfire air injection is generally in the range of 60% to 100%. Nitrogen bearing compounds that evolve from the waste react with oxygen in and downstream of the overfire air injection zone, forming significant additional NO.sub.x. Because of the low combustion temperatures in and downstream of the overfire air injection, most of the NO.sub.x formed in this zone is by the oxidation of nitrogen bearing compounds (less than about 10% are formed in this zone by the oxidation of molecular nitrogen). Based on measurements by the inventors, typical mass burn operations would result in about 30% of the total NO.sub.x formed on the stoker and about 70% in and downstream of the overfire air injection.
In most cases, a boiler is an integral part of the combustor to recover the heat generated by MSW combustion. In some cases, cooled flue gases from downstream of the boiler are recirculated back into the combustion zone to reduce oxygen concentration and to lower combustion temperatures and thus are believed to decrease oxides of nitrogen formation. A disadvantage of flue gas recirculation (FGR) is generally a higher concentration of products of incomplete combustion within the flue gases and within the stack gases because of reduced combustion efficiency.
U.S. Pat. No. 3,781,162 teaches an apparatus for mixing recirculated flue gases with combustion air before the gases reach an igniter. The '162 patent discloses combustion without recirculating vitiated air from over a burnout grate for overfiring. The '162 patent teaches neither fluid swirling in the combustion chamber nor injecting fuel above a stoker grate.
U.S. Pat. No. 3,938,449 discloses a waste disposal facility which uses a rotary kiln that differs from a stoker. The rotary kiln includes a hollow, open-ended circular tube body mounted for rotation about its circular axis. Hot flue gases are recirculated to dehydrate the waste material and remove oxygen. The '449 patent does not disclose fluid swirling in the combustion chamber or fuel injection downstream of the primary waste combustion zone.
U.S. Pat. No. 4,336,469 teaches a method of operating a magnetohydrodynamic (MHD) power plant for generating electricity from fossil fuel. The MHD combustor has a first stage which operates substoichiometrically, second stage natural gas injection, and third stage air injection for complete combustion. The '469 patent does not disclose the use of vitiated air from the combustor for overfiring and does not disclose fluid swirling within the combustion chamber. The '469 patent discloses a dwell chamber downstream of the MHD generator for reducing nitrogen oxides, but does not disclose nitrogen bearing compound decomposition.
U.S. Pat. No. 4,672,900 teaches a tangentially-fired furnace having injection ports for injecting excess air above a fireball of the combustion chamber to eliminate the flue gas swirl as the flue gas flows into a convection section. The furnace uses pulverized coal as a fuel. Secondary air is tangentially injected into the furnace and swirls in the direction opposite of the flue gas swirl. The '900 patent does not suggest the use of recirculated vitiated air from the main combustor for overfiring, fluid swirling within the combustion chamber, or fuel injection downstream of the primary combustion zone.
U.S. Pat. Nos. 4,013,399, 4,050,877 and 3,955,909 teach reduction of gaseous pollutants in combustion flue gas. The '909 patent discloses two-stage combustion within a combustion chamber. Heat removal occurs in the first, second or both combustion stages to reduce nitrogen oxides. Secondary combustion air is injected or diffused through tubes into the stream of gaseous combustion products flowing from a primary combustion chamber to promote mixing and complete combustion without an excessive amount of secondary air.